E. f-number – a measure of the amount of light gathered by a lens or mirror.

A high number gathers less light, so (with telescopes) 5.6 may be better than 8. It is said to be "faster". By capturing more light, it can produce brighter images or images that take less exposure time. The lower number also has a wider field of view. In cameras, you may see f/ratio settings like: f/1.2, 2, 2.8, 4, 5.6, 8, 11, 16, 22. In telescopes, you may find: f/4, 5.4, 6, 8, 11. In cameras, "stopping down" to higher f-numbers produces a greater "depth of field" but forces a longer exposure time.

$ f/ratio = \frac{f.l.}{diameter} $

F. Magnification – number of times a telescope/microscope increases the size of an image

$ M=\frac{f_1}{f_2} $

where $ f_1 $ is the Focal Length of the Primary Mirror of a reflecting telescope (or the Objective Lens of a refracting telescope or Microscope) and $ f_2 $ is the Focal Length of the Eyepiece.